This invention relates to organic electroluminescent (EL) devices. More specifically, this invention relates to stable and saturated red emission EL devices.
Organic light emitting diodes (OLEDs), also known as organic electroluminescent devices, are a class of electronic devices which emit light in response to an electrical current applied to the device. Organic EL devices are also known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat panel displays have been contemplated. While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, xe2x80x9cDouble Injection Electroluminescence in Anthracenexe2x80x9d, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 xcexcm). Consequently, operating voltages were very high, often  greater than 100 V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g.  less than 1.0 xcexcm) between the anode and the cathode. Herein, the organic EL element encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltages. In a basic two-layer EL device structure, described first in commonly-assigned U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole transport layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron transport layer. The interface between the two layers provides an efficient site for the recombination of the injected hole/electron pair and the resultant electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in commonly-assigned U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
For the production of full-color EL display panel, it is necessary to have efficient red, green and blue (RGB) EL materials with proper chromaticity and sufficient luminance efficiency. The guest host doped system offers a ready avenue for achieving such an objective, mainly because a single host with optimized transport and luminescent properties may be used together with various guest dopants leading to EL of desirable hue.
A doped EL system based on the principle of guest host energy transfer to effect the spectral shift from tris(8-hydroxyquinolinato)aluminum (Alq) to the dopant molecules has been disclosed by Tang et al [commonly-assigned U.S. Pat. No. 4,769,292]. Alq is a suitable host for red EL emitters since its emission at 530 nm is adequate to sensitize guest EL emission in the red spectral region. The preferred dopants chosen to provide the red emission in this prior art were 4-(dicyanomethylene)-2-methyl-6-(pdimethylaminostyryl)-4H-pyran (DCM) and the julolidyl derivatives DCJT and DCJTB. 
These molecules generally have a high photoluminescence (PL) quantum yield and the position of the emission maxima can be readily shifted by certain modification of the DCM structure. However, the luminance efficiency of the Alq/DCM system is compromised by two factors. First, the spectral bandwidth of the emission is rather broad. As a result, a suitable red hue can be obtained only with the dominant emission in the deep red region. The broadness of the emission band yields a significant portion of photons in the long wavelength spectral region where the eye is not sensitive resulting in a loss of luminance efficiency. Second, the EL efficiency of the guest host system is highly dependent on the concentration of the guest in the host matrix. The concentration quenching effect, presumably due to the aggregation of guest molecules, is relatively strong in the Alq/DCM system. A further loss in luminance efficiency would result if a concentrated guest host system is necessary to provide for an adequate red hue in the EL emission.
It is an object of the present invention to provide an improved red emitting organic EL device.
It is an another object of the present invention to provide a red emitting device for full color organic EL device.
These objects are achieved by an organic EL device, comprising an anode and a cathode, and at least one organic luminescent medium including a compound of the formula: 
wherein:
R1, and R2 are individually alkyl of from 1 to 20 carbon atoms, aryl, substituted aryl, carbocyclic and other heterocyclic systems; and R1, and R2 can be connected to form 5 or 6 member ring systems; and R3, and R4 are individually hydrogen; alkyl of from 1 to 10 carbon atoms, and a branched or unbranched 5 or 6 member substituent ring connecting with R1, R2 respectively; and R5 and R6 are individually hydrogen; alkyl of from 1 to 20 carbon atoms; aryl and heteroaryl of from 5 to 24 carbon atoms; and R6 can be connected with R5 to form a branched or unbranched 5 or 6 member carbocyclic ring.
It is a feature of the present invention that EL devices made using compounds in accordance with the above structure produce an improved and highly saturated red hue.